Have you seen the thread with crowd-sourced book recommendations for gifted kids age 3-5?

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his reading level is in 2nd grade... how do I know which book is in which reading level?
Lexile levels can be of some help as a general guideline, and Lexile offers a Find-A-Book feature. To find books at a 2nd grade level using the Find-A-Book feature, you could enter that your child is in 2nd grade and finds the books to be just right, then select your child's interests and see the list of recommendations at that level.

Because children vary, parents often pre-read books when selecting them for their children. This provides the added benefit of being able to discuss the book together.